Semiconductor laser

ABSTRACT

The invention has such a double hetero structure ( 11 ) that an active layer ( 3 ) is sandwiched by an n-type clad layer ( 2 ) and a p-type clad layer ( 4 ) on a semiconductor substrate ( 1 ) made of GaAs. In the p-type clad layer ( 4 ), for example, an n-type current constriction layer ( 6 ) consisting of at least two layers is provided in such a configuration that a first layer ( 6   a ) thereof closer to the active layer is made of a material having almost the same refractive index as the p-type clad layer and a second layer ( 6   b ) thereof farther from the active layer is made of a material having a smaller refractive index than the first layer ( 6   a ). By this configuration, a self-excitement type and high-power semiconductor laser can be obtained which operates in a stable manner up to a high power without generating a kink while being self-excited at a low power. Another embodiment of the invention comprises a current constriction layer having an n-type in which a stripe trench is formed in the p-type clad layer, and a light confinement layer having a smaller refractive index than the p-type clad layer is formed at the current constriction layer facing the active layer, so as to be of a p-type or non-doped type. By this configuration, a semiconductor laser can be obtained which operates up to a high power without generating a kink.

FIELD OF THE INVENTION

[0001] The present invention relates to a semiconductor laser used as alight source for an optical disk device such as a CD-ROM, a CD-R, or aDVD-ROM, a high-definition laser beam printer (LBP), a laser pointer, orthe like. More specifically, it relates to such a semiconductor laserthat can oscillate at a high power without generating a kink byconstricting a current by use of the current constriction layer as wellas confining the light as much as possible, or that can oscillate at ahigh power without generating a kink while being self-excited at a lowpower by confining the light as well to some extent.

BACKGROUDN OF THE INVENTION

[0002] A self-alignment type semiconductor laser provided with a lightconfinement effect due to a current constriction layer has such aconstruction as shown in FIG. 4 as an example. That is, as shown in FIG.4, on a substrate 21 made of, e.g., an n-type GaAs are sequentiallygrown epitaxially an n-type clad layer 22 made of, e.g., n-typeAl_(0.6)Ga_(0.4)As, an active layer 23 made of non-dopedAl_(0.2)Ga_(0.8)As, a p-type first clad layer 24 a made of p-typeAl_(0.6)Ga_(0.4)As, an etching stopping layer 25, a current constrictionlayer 26 made of, e.g., n-type Al_(0.7)Ga_(0.3)As, a p-type second cladlayer 24 b made of p-type Al_(0.6)Ga_(0.4)As, and a p-type contact layer27 made of GaAs, on the top surface of which is formed a p sideelectrode 28 and on the back surface of the GaAs substrate 21 is formedan n side electrode 29, so that the resultant wafer is subdivided intochips by cleavage or the like to thereby form a semiconductor laser (LD)chip having a construction shown in FIG. 4.

[0003] This construction employs, to use the laser for write-inoperations or the like, a method for disposing the current constrictionlayer near the active layer or enlarge the mixed-crystal ratio of Al inthe current constriction layer to provide an effective difference inrefractive index in order to enhance the light confinement effect, thusoscillating a laser at a high power.

[0004] Also, to use the laser for read-out operations or the like, sucha method is required to be employed that the noise is reduced at a lowpower and, for self excitement, the mixed-crystal ratio of Al in thecurrent constriction layer is reduced or the current constriction layeris disposed distant from the active layer to thereby relax the lightconfinement effect in order to spread the light, thus enabling forming asupersaturating absorption layer outside a current implanting region inthe active layer.

[0005] As mentioned above, in order to enhance the light confinementeffects and oscillate a laser at a high power, the current constrictionlayer must be disposed near the active layer as much as possible or themixed-crystal ratio of Al in the current constriction layer made of anAlGaAs-based compound semiconductor must be enlarged to thereby reducethe refractive index. If the mixed-crystal ratio of Al is enlarged,however, the exposed surface of the current constriction layer after astripe trench is formed therein easily corrodes because Al is veryeasily oxidized, thus suffering from a problem that a clean mono-crystalsemiconductor layer cannot easily be grown when a semiconductor layer isgrown again. Although by, in particular, forming beforehand a protectivelayer such as made of GaAs on the top-most surface of the currentconstriction layer, thermal etching can be carried out before there-growing, thus providing a clean layer, the side walls of the stripetrench cannot be cleaned in such a way, so that the semiconductor layeris liable to be polycrystallized to thereby flow a leakage current andso increase the threshold current value, thus leading to a problem of anincreased electric resistance due to polycrystallization and also a risein the operation current.

[0006] Since the current constriction layer, on the other hand, has aconductivity type different from that of its surrounding clad layer tothereby prevent a current flow by the reverse-biased pn junction, thepn-junction portion has a depletion layer formed thereon due to thereverse biasing, so that as shown in FIG. 5, if the current constrictionlayer 26 is formed too close to the active layer 23, the depletion layer(refer to C in FIG. 5) reaches the active layer 23. If the depletionlayer C reaches to the active layer 23, as shown in FIG. 5, a current Iflows to the portion of the current constriction layer 26 where nostripe trench is formed to disable from constricting the current, thusleading to a problem that the invalid current flows through the activelayer.

[0007] Although to use a laser for both write-in and read-outoperations, on the other hand, the laser must oscillate at a high powerwithout generating a kink while being self-excited at a low power tothereby reduce the noise, as mentioned above, there is a trade-offrelationship between self-excitement at a low power and obtaining a highpower and, therefore, both requirements cannot be satisfied at the sametime, so that a self-excitement type semiconductor laser, which has alarge fluctuation in power, suffers from a phenomenon called a kink thatthe power drops during the process of increasing the operation current,leading to a problem of difficulty in obtaining of a high power. Asemiconductor laser for obtaining a high power, on the other hand,cannot be self-excited, leading to a problem that the noise cannot besuppressed.

[0008] In view of the above, it is an object of the invention to providea self-excitement type, high-power semiconductor laser that can operatein a stable manner even at a high power without generating a kink whilebeing self-excited at a low power.

[0009] It is another object of the invention to provide a high-powersemiconductor laser which can operates in a stable manner even at a highpower with no kink generated without enlarging so much the mixed-crystalratio of Al in the current constriction layer and also with preventing adepletion layer due to a pn junction of the current constriction layerfrom reaching the active layer.

SUMMARY OF THE INVENTION

[0010] A semiconductor laser according to the invention comprises anactive layer sandwiched by n-type and p-type clad layers in such aconstruction that either one of the above-mentioned clad layers isprovided with a current constriction layer consisting of at least twolayers for current confinement and light confinement, the first layer ofthe current constriction layer closer to the above-mentioned activelayer has a conductivity type different from that of the clad layerprovided with the current constriction layer and being made of amaterial having almost the same refractive index as that of the cladlayer and the second layer of the above-mentioned current constrictionlayer farther from the active layer being made of a material having arefractive index smaller than that of the first layer.

[0011] By this construction, the first layer closer to the active layerhas almost the same refractive index as that of the clad layer tothereby eliminate the light confinement effect, thus serving as a layeronly for constricting the current. The second layer farther from theactive layer, on the other hand, has a smaller refractive index than thefirst layer and has the light confinement effect. In this case, thelight is emitted from the current implanting region constricted by thefirst layer closer to the active layer and then the second layerconfines the light in the light emitting region from which the light wasoscillated, so that the light emitting region in which the light isconfined becomes larger than the region to which a carrier is implantedto emit the light thereby permitting thus enlarged portion of the activelayer to act as an supersaturating absorption layer. Accordingly, thelight can be confined in an enhanced manner while reserving thatsupersaturating absorption layer, to sufficiently confine the lightduring self-excitement by use of the supersaturating absorption layer,thus obtaining a high power without generating a kink. Note here thatthe second layer does not have to be different in conductivity type fromthe surrounding clad layer but may have the same conductivity type ifthe first layer can confine the current sufficiently.

[0012] In the event that, for example, AlGaAs based compoundsemiconductor for emitting the infrared light or InGaAlP based compoundsemiconductor for emitting the red light forms a double heteroconstruction in which the active layer is sandwiched by n-type andp-type clad layers having a larger band gap than the active layer, thefirst layer of the current constriction layer is made of a materialhaving almost the same composition as that of the clad layer of theAlGaAs based or InGaALP based compound semiconductor, and the secondlayer is made of a material having an enlarged mixed-crystal ratio of Alto thereby provide an enlarged difference in refractive index, so thatthe larger the mixed-crystal ratio of Al or the closer to the activelayer, the more the layer has the light confinement effect.

[0013] It is preferably that the first and second layers are so formedas to function mainly as a current confinement layer and a lightconfinement layer respectively and also the stripe trench provided tothe first layer is so formed as to be smaller in width than thatprovided to the second layer, because the supersaturating absorptionlayer can be reserved securely. That is, if the stripe trench is formednot perpendicular to the surface of the semiconductor layer but isformed to have an inclined surface with respect to that, the stripewidth of the first layer closer to the active layer is made smaller thanthat of the second layer. In this case, however, the two stripe trenchesmay be etched in different patterns to form the first layer narrowerthan the second layer.

[0014] Even if the stripe trenches are formed so as to have an inclinedsurface with respect to the width direction of the current constrictionlayer and the inclined surface of the first layer is formed so as tohave a smaller inclination angle than that of the second layer, thestripe trench of the first layer has an even smaller width than thesecond layer, so that the supersaturating absorption layer can beexpanded. Here, the inclination angle of the inclined surface refers toan angle θ of the side wall of the stripe trench with respect to thegrowing surface of the semiconductor layer (see FIG. 1(b)).

[0015] The semiconductor laser according to the invention may haveanother construction that has a double hetero structure in which theactive layer is sandwiched by n-type and p-type clad layers and eitherone of these clad layers is provided with a current constriction layerwith a stripe trench having a different conductivity type from that ofthis clad layer and also at the current constriction layer on the sideof the active layer a light confinement layer having s smallerrefractive index than this clad layer is formed without doping or isformed so as to have the same conductivity type as this clad layer.

[0016] By this construction, the light confinement layer having asmaller refractive index is formed without doping or so as to have thesame conductivity type as the clad layer, so that the only the lightconfinement effect can be obtained without forming a reverse-biased pnjunction. Accordingly, the light confinement layer can be disposed closeto the active layer unlimitedly. On the side of that light confinementlayer opposite to the active layer, on the other hand, a currentconstriction layer having a different conductivity type from that of theclad layer is provided to thereby inhibit a current by thereverse-biased pn junction. A depletion region due to this reversebiasing would spread to the light confinement layer but can becontrolled not to reach the active layer by adjusting the thickness ofthe light confinement layer, thus preventing a leakage current fromflowing therethrough. As a result, it is possible to enhance the lightconfinement effect while sufficiently preventing the leakage currentfrom flowing, thus providing stable operations up to a high power.

[0017] If the current constriction layer is formed of a semiconductorlayer having the same refractive index as the light confinement layer,it is possible for this light confinement layer, even if it is thin, toprevent a depletion layer due to the reverse biasing from reaching theactive layer while sufficiently confining the light together with thecurrent constriction layer.

BRIEF DESCRIPTION OF THE DRAWINGS

[0018] FIGS. 1(a) to 1(b) are cross-sectional views explaining oneembodiment of a semiconductor laser according to the present invention.

[0019]FIG. 2 is a cross-sectional view explaining a variant of thesemiconductor laser of FIG. 1.

[0020]FIG. 3 is a cross-sectional view explaining another embodiment ofthe semiconductor laser according to the present invention.

[0021]FIG. 4 is a cross-sectional view explaining a conventionalself-exciting type semiconductor laser.

[0022]FIG. 5 is a cross-sectional view explaining a problem with a casewhere a current constriction layer is disposed too close to an activelayer in a construction of FIG. 4.

DETAILED DESCRIPTION

[0023] The following will describe embodiments of a semiconductor laseraccording to the present invention with reference to the drawings. Asshown in a cross-sectional view of one embodiment thereof in FIGS. 1(a)and 1(b), the semiconductor laser according to the present invention hassuch a double hetero structure 11 on a semiconductor substrate 1 madeof, e.g., n-type GaAs that an active layer 3 is sandwiched by n-type andp-type clad layers 2 and 4. Further, for example, the p-type clad layer4 is provided with a current constriction layer 6 consisting of at leasttwo layers having a conductivity type (e.g., n-type) different from theconductivity type (e.g., p-type) of this clad layer 4, in such aconfiguration that a first layer 6 a of the current constriction layer 6which layer is closer the active layer 3 is made of a material havingalmost the same refractive index as that of the p-type clad layer 4 anda second layer 6 b of the current constriction layer 6 which layer isfarther from the active layer 3 is made of a material having a smallerrefractive index than the first layer 6 a.

[0024] In such a case as shown in FIG. 1(a), the double hetero structure11 has such a stacked-layer configuration that comprises the n-type cladlayer 2 made of, e.g. n-type Al_(x)Ga_(1-x)As (0.3≦x≦0.8, e.g. x=0.6),the active layer 3 made of non-doped, n-type, or p-type Al_(y)Ga_(1-y)As(0≦y≦0.3, e.g. y=0.15), and a p-type first clad layer 4 a made of p-typeAl_(x)Ga_(1-x)As. Specifically, in order to determine the material ofthe active layer 3 based on a band gap corresponding to a desired lightemission wavelength to confine a carrier in the active layer 3, thisdouble hetero structure 11 is sandwiched by the clad layers 2 and 4which have a larger band gap than that material. Therefore, depending onthe desired wavelength, in place of an AlGaAs based compound, an InGaAlPbased compound or any other semiconductor may be used. Note here thatbesides being the bulk layer, the active layer 3 may be of amultiple-quantum construction of AlGaAs—AlGaAs (AlGaAs—GaAs) thatalternately stacks a well layer made of Al_(0.1)Ga_(0.9)As or GaAs and abarrier layer made of Al_(0.3)Ga_(0.7)As.

[0025] The p-type clad layer 4 is divided into a first clad layer 4 aand a second clad layer 4 b with the current constriction layer 6 formedtherebetween via the etching stop layer 5. The etching stop layer 5 isprovided for stopping further etching when a stripe trench 6 c is formedin the current constriction layer 6 and only needs to be formed by sucha composition as to enable local etching of the current constrictionlayer 6, e.g. a non-doped or p-type composition of Al_(a)Ga_(1-a)As(0≦a≦1, a≠z, r) or InGa_(1-b)Al_(b)P (0≦b≦0.5).

[0026] The current constriction layer 6 is divided into a first layer 6a made of, e.g. n-type Al_(z)Ga_(1-z)As (0.3≦z≦0.8, z is nearly equal tox) and a second layer 6 b made of Al_(r)ga_(1-r)As (0.4≦r≦1, x<r, z<r),with the stripe trench 6 c formed through both layers (which trenchappears to be extending perpendicular to the paper surface because thefigure shows a cross-sectional view in a direction perpendicular to thestripe).

[0027] This first layer 6 a is made of a material having almost the samecomposition as the p-type clad layer 4 and so has little lightconfinement effect. However, as can be seen from the fact that it isformed as an n-type layer in the p-type clad layer 4, this first layer 6a has a different conductivity type from the clad layer 4 provided withthe current constriction layer 6 and so serves to inhibit a currentflow, thus permitting a current to flow only through the portion wherethe stripe trench 6 c is formed. That is, it has an action to narrow acurrent only to the portion of the stripe trench 6 c. The second layer 6b, on the other hand, has a larger mixed-crystal ratio of Al of theAlGaAs based compound than the first layer 6 a and is made of a materialhaving a smaller refractive index so as to have a light confinementeffect and, at the same time, is formed to be an n-type compound so asto have a current constriction effect in corporation with the firstlayer 6 a. In an example shown in FIG. 1(a), although the second layer 6b is also formed as an n-type layer, the first layer 6 a alone canconstrict the current if it has a width of about 0.2 μm, in which casethe conductivity type of the second layer 6 b is not limited.

[0028] The current constriction layer 6 is thus comprised of at leasttwo layers in such a configuration that the first layer 6 a closer tothe active layer 3 only has a current confining effect without confiningthe light, to permit the second layer 6 b to have the light confinementeffect. Besides, although the above-mentioned stripe trench 6 c can beetched so as to have almost perpendicular side walls of its own byadjusting the etching conditions, the inclination angle θ of the sidewalls is decreased if the ratio of a hydrogen peroxide component isdecreased in, for example, a sulfuric acid-based etching solution, sothat as shown in FIG. 1(b), the side walls are formed so as to have aninclined surface with respect to the surface of the stackedsemiconductor layers. Accordingly, the width of the stripe trench 6 c isformed so as to have a width B in the second layer 6 b larger than awidth A in the first layer 6 a.

[0029] On the current constriction layer 6 and also on the etching stoplayer 5 exposed through the stripe trench 6 c is grown a p-type secondclad layer 4 b having the same composition as the p-type first cladlayer 4 a, on which is further provided a contact layer 7 made of p-typeGaAs, on the surface of which is provided a p-type electrode 8 and onthe back surface of semiconductor substrate 1 is provided an n-typeelectrode 9, so that the structure can be subdivided into chips bycleavage and the like, thus obtaining a semiconductor laser having theconstruction shown in FIG. 1. Although in the above-mentioned examplethe n-type GaAs substrate has been employed, a p-type substrate may alsobe used to obtain the same results by reversing the conductivity typesof all those semiconductor layers and also the current constrictionlayer may be formed below the active layer.

[0030] To manufacture the semiconductor laser shown in FIG. 1(a), firston the semiconductor substrate 1 made of n-type GaAs, by using an MOCVDor MBE method, the n-type clad layer 2 made of n-type Al_(0.6)Ga_(0.4)Aswith a thickness of about 1 μm, the active layer 3 made of non-dopedAl_(0.15)Ga_(0.85)As with a thickness of about 0.1 μm, the first cladlayer 4 a made of p-type Al_(0.6)Ga_(0.4)As with a thickness of about0.15 μm, the etching stop layer 5 made of non-dopedIn_(0.5)Ga_(0.4)Al_(0.1)P with a thickness of about a few tens ofnano-meters, the first layer 6 a of the current constriction layer 6made of n-type Al_(0.6)Ga_(0.4)As with a thickness of about 0.3 μm, thesecond layer 6 b of the current constriction layer 6 made of n-typeAl_(0.7)Ga_(0.3)As with a thickness of about 0.3 μm, and an oxidationpreventing layer made of n-type GaAs (not shown) with a thickness ofabout 0.03 μm are stacked sequentially.

[0031] Next, this device in process is masked with a photo-resist excepta formation-reserved portion of the stripe trench 6 c and etched with,for example, a sulfuric acid-based solution to form the stripe trench 6c, thus forming the stripe trench 6 c. This solution cannot etchIn_(0.5)Ga_(0.4)Al_(0.1)P, so that etching is stopped at the etchingstop layer 5 to prevent the p-type first clad layer 4 a from beingetched, thus etching only the current constriction layer 6 to apredetermined width. In this case, by decreasing the hydrogen peroxidecomponent of the sulfuric acid-based solution, the side walls of thestripe trench 6 c has a smaller inclination angle θ, thus obtaining amoderately inclined surface.

[0032] Then, the device in process is put in a reactive vessel such as aMOCVD apparatus again to conduct thermal cleaning to thereby evaporatethe oxidation preventing layer made of GaAs (not shown), thus stackingboth the p-type second clad layer 4 b made of p-type Al_(0.6)Ga_(0.4)Asand the contact layer 7 made of p-type GaAs to about 1 μm. Then, therespective p-type and n-type electrodes 8 and 9 are formed by, forexample, evaporation and then subdivided into chips, thus obtaining asemiconductor laser chip shown in FIG. 1(a).

[0033] According to the present invention, the current constrictionlayer 6 is, as mentioned above, comprised of at least two layers in sucha configuration that the first layer 6 a thereof closer to the activelayer 3 has only the current confinement effect without confining thelight and the second layer 6 b thereof acts to confine the light.Accordingly, as shown in an expanded portion of the current constrictionlayer of FIG. 1(b), a portion A where a carrier is confined in theactive layer 3 to thereby emit the light does not agree with a portion Bwhere the light is confined by the second layer 6 b of the currentconstriction layer 6 (which light is actually confined by the secondlayer 6 b as distant from the active layer 3, so that the portion Bspreads laterally a little), so that the light confinement region Bbecomes wider than the other. Accordingly, even if the light is confinedin an enhanced manner, a non-overlapping portion 3 b of the lightconfinement region B and the light emitting region A due to the carrierof the active layer 3 is supersaturated, to provide a repetitive regionacting as a light emitting and light non-emitting regions alternatelyduring a low-power operation, thus being self-excited. During ahigh-power operation, on the other hand, it always acts as a lightemitting region, to oscillate a light in a stable manner throughout theoperation region, thus obtaining a high power without a kink.

[0034] Although in the above-mentioned example, the side walls of thestripe trench 6 c of the current constriction layer 6 has been formed tohave almost the same inclination angle through the two layers, as shownin FIG. 2, the etching conditions may be changed to form it with amoderate inclination angle of the first layer 6 a and a steepinclination angle of the second layer 6 b in order to enlarge thenon-overlapping portion of the above-mentioned light emitting region Aand light confinement region B, thus increasing the width of thesupersaturating absorption layer. As a result, the device can beself-excited easier at a low power, thus enhancing the light confinementeffect. Also, although not shown, the first layer 6 a and the secondlayer 6 b may be patterned in etching thereof to form the stripe trench6 c, to thereby change the width or, similarly, by makingself-excitement easier to occur, the light confinement effect can beenhanced. The other elements in FIG. 2 are not explained here becausethey are indicated by the same reference numbers as those in FIG. 1(a).

[0035] In the self-excitement type semiconductor laser according to theinvention using a current constriction layer to have a light confinementeffect, current constriction and light confinement operations can beperformed separately from each other, so that even if the light isconfined in an enhanced manner, an supersaturating absorption layer canbe reserved, to thereby satisfy both requirements of self-excitement anda high power operation without a kink. As a result, such a semiconductorlaser can be obtained that has low-noise characteristics free the noisedue to the return light and also has high-power characteristics.

[0036] A high-power semiconductor laser according to the presentinvention comprises, as shown in a cross-sectional view of itsembodiment of FIG. 3, has such a double hetero structure 11 that on thesemiconductor substrate 1 made of, e.g., n-type GaAs the active layer 3is sandwiched by n-type and p-type clad layers 2 and 4(4 a). In, forexample, the p-type clad layer 4 (4 a and 4 b) is provided the currentconstriction layer 6 in which the stripe trench 6 c is formed which hasa different conductivity type (e.g., n-type) from the conductivity type(e.g., p-type) of that clad layer 4, on the active layer 3 side of thatcurrent constriction layer 6 is formed a light confinement layer 10which has a smaller refractive index than the p-type clad layer and alsowhich has the same conductivity type as that p-type clad layer 4 or isnon-doped.

[0037] The double hetero structure 11 is the same as that of theabove-mentioned case shown in FIG. 1(a) except that the currentconstriction layer does not consists of two layers but comprises thelight confinement layer 10 and the current constriction layer 6.

[0038] In the case shown in FIG. 3, both the light confinement layer 10and the current constriction layer 6 are made of Al_(s)Ga_(1-x)As(0.4≦s≦0.8, x<s) and have the stripe trench 6 c formed therein (whichappears to extend perpendicular to the paper surface because the figureshows the cross-sectional view in a direction perpendicular to thestripe). That is, it is formed to have a larger mixed-crystal ratio s ofAl than the clad layer 4 to thereby have a reduced refractive index,thus acting to confine the light to the side of the active layer 3.

[0039] On the other hand, the light confinement layer 10 is formed tohave the same conductivity type as the surrounding clad layer, i.e.p-type or non-doped type in the case shown in FIG. 3, while the currentconstriction layer 6 is formed to have the opposite conductivity type,i.e. n-type. Accordingly, at the boundary between this currentconstriction layer 6 and the light confinement layer 10 is formed areverse-biased pn junction surface to inhibit a current flow, so that acurrent flows only through a portion where the stripe trench 6 c isformed, thus narrowing the current. If a reverse bias is applied to thispn junction surface, as indicated by a broken line in FIG. 3, thedepletion layer C is formed on both sides of the pn junction. The lightconfinement layer 10 is so formed that this depletion layer C has such athickness as not to reach the active layer 3. If the light confinementlayer 10 is too thick, the current injecting region is liable to spreadfrom the stripe trench, which is not preferable. For example, the lightconfinement layer 10 is formed to a thickness of about 0.05 - 0.3 μm andthe current constriction layer 6, to a thickness of about 0.2 - 0.5 μm.

[0040] Explanation of the other structures and the manufacturingprocedure is omitted here because they are the same as theabove-mentioned example shown in FIG. 1(a) and indicated by the samereference numerals.

[0041] According to this embodiment, besides the current constrictionlayer 6, on the side of the active layer, the light confinement layer 10having a light-confinement action is formed which has a non-doped typeor the same conductivity type as the clad layer 4 surrounding thatcurrent constriction layer 6 and also which has a smaller refractiveindex than the clad layer. Accordingly, even if the light confinementlayer 10 comes close to the active layer 3, the depletion layer does notbreak through into the active layer, thus enabling enhancing the lightconfinement effect due to the proximity to the active layer 3. As aresult, without reducing the refractive index by unnecessarily enlargingthe mixed-crystal ratio of Al of the current constriction layer 6, thelight can be confined sufficiently to improve the crystallinity, thusimproving the light emission characteristics including a reduced currentvalue and also enhancing the light confinement effect for stableoperations up to a high power.

[0042] By this embodiment employing an AlGaAs based compound or InGaAlPbased compound semiconductor, in the self-alignment type semiconductorlaser which provides a high power by confining the light, since thelight confinement layer besides the current constriction layer is formedto have a non-doped type or the same conductivity type as thesurrounding clad layer, the light confinement layer can be disposedclose to the active layer arbitrarily to enhance the light confinementeffect without enhancing it by unnecessarily enlarging the mixed-crystalratio of Al. As a result, a stable, high-power semiconductor laser canbe obtained which does not generate a kink.

[0043] Although preferred examples have been described in some detail itis to be understood that certain changes can be made by those skilled inthe art without departing from the spirit and scope of the invention asdefined by the appended claims.

What is claimed is:
 1. A semiconductor laser comprising: an n-type cladlayer; a p-type clad layer; an active layer sandwiched by said n-typeclad layer and said p-type clad layer; and a current constriction layerfor current confinement and light confinement consisting of at least twolayers which is disposed in either of said n-type clad layer and saidp-type clad layer, wherein a first layer of said current constrictionlayer closer to said active layer has a different conductivity type froma conductivity type of either of said clad layers in which said currentconstriction layer is provided and is made of a material having almostthe same refractive index as said clad layer, and wherein a second layerof said current constriction layer farther from said active layer ismade of a material having a smaller refractive index than said firstlayer.
 2. The semiconductor laser of claim 1 , wherein said first layerof said current constriction layer is formed to function mainly as acurrent confinement layer and said second layer thereof is formed tofunction mainly as a light confinement layer and a width of a stripetrench for injecting current provided in said first layer is smallerthan a width of a stripe trench provided in said second layer.
 3. Thesemiconductor laser of claim 2 , wherein said stripe trench is formed soas to have an inclined surface with respect to a width-direction of saidcurrent constriction layer, so that a width of said stripe trench forinjecting current provided in said first layer may be smaller than awidth of said stripe trench provided in said second layer.
 4. Thesemiconductor laser of claim 3 , wherein said inclined surface of saidfirst layer has a smaller inclination angle than said second layer. 5.The semiconductor laser of claim 2 , wherein said stripe trench in saidfirst layer and said stripe trench in said second layer are provided indifferent steps, so that the width of said stripe trench provided insaid first layer may be smaller than the width of said stripe trenchprovided in said second layer.
 6. A semiconductor laser comprising: ann-type clad layer; a p-type clad layer; an active layer sandwiched bysaid n-type clad layer and said p-type clad layer; a currentconstriction layer, formed in either of said n-type and p-type cladlayers, in which a stripe trench is formed, said current constrictionlayer having a different conductivity type from said clad layer, and alight confinement layer formed at said current constriction layer facingsaid active layer and having a smaller refractive index than said cladlayer, wherein said light confinement layer is formed to have anon-doped type or the same conductivity type as said clad layer in whichsaid light confinement layer is provided.
 7. The semiconductor laser ofclaim 6 , wherein said current constriction layer is formed of asemiconductor layer having the same refractive index as said lightconfinement layer.
 8. The semiconductor laser of claim 6 , wherein athickness of said light confinement layer is set so that a depletionlayer formed between said current constriction layer and said lightconfinement layer may not reach said active layer and also that saidcurrent constriction layer may be close to said active layer.
 9. Thesemiconductor laser of claim 8 , wherein said light confinement layer isformed to have a thickness of 0.05-0.3 μm and said current constrictionlayer is formed to have a thickness of 0.2 - 0.5 μm.